The present invention relates to power amplifiers for use in radio transmitters and more particularly to high efficiency amplifiers suitable for use in high power applications.
It is important for power amplifiers used in radio transmitters to be as efficient as possible in order to reduce amounts of wasted energy and minimize the requirements for cooling associated with these amplifiers. In particular, high efficiencies have been achieved in the past through the use of "class D" type amplifiers in which the RF carrier assumes a bi-level square wave shape with the signal voltage alternating between high and low voltage levels. This wave shape allows the efficiency of the amplifier to be increased since it limits the extent to which the amplification devices are simultaneously subject to substantial amounts of voltage potential and current flow. Unfortunately, this waveshape is also associated with significant amounts of high frequency harmonics which in wideband applications must be filtered off prior to the signal being supplied to the antenna for broadcast, leading to the dissipation of appreciable amounts of energy.
In an alternative to class D operation, sometimes referred to as "class G" operation, amplifiers are constructed using separate transistors connected to separate power supplies which are coupled together so as to be independently operative during different parts of the RF carrier cycle. However, in class G operation the transistors operate in the linear mode producing a continuous sine wave type output which is therefore subject to the same type of efficiency limits (although to a lesser degree) as class B amplifiers . Class D and class G amplifiers are naturally limited in efficiency by their inherent designs and the waveshapes of the carrier signals which they amplify. More specifically, these amplifiers are theoretically limited to efficiencies no higher than approximately 70% in the case of class G amplifiers and approximately 81% in the case of wideband type class D amplifiers.
It is therefore an object of the present invention to provide a wideband amplification system for use at high power levels having the highest possible efficiency.
It is another object of the present invention to provide a wideband power amplifier in which the waveshape of the RF carrier is optimized for high efficiency and the harmonics content associated with the wave shape is minimized prior to filtering.
It is a further object of the present invention to produce a high power and high efficiency wideband amplification system which is relatively simple and economic to construct yet is capable of achieving theoretical efficiencies greater than 90%.